Referring to FIG. 1, a conventional method of cooling strip metal in a continuous annealing furnace is shown. The strip metal 1 is sequentially wound partially around a series of spaced cooling rolls 2 in such a way that the strip follows a serpentine path, and is cooled over the areas where it contacts the rolls 2. This method has great advantages. Firstly, it poses no problems about the shape of the surface of the strip 1. Secondly, the strip can be processed in an economical manner. However, it is likely that the standard shape of the strip 1 will be deformed, depending upon the manner in which it contacts with the cooling rolls 2. Specifically, strip metal cooled in this way usually shows a center buckle, or edge wave, of the irder of 0.1%. Therefore, some portions of the strip make good contact with cooling rolls and are rapidly cooled, while the others make poor contact with them. This creates an uneven temperature distribution across the width of the strip. As a result, thermal stresses are produced, deforming the strip from its standard shape.
In an attempt to reduce the possibility of deformation of the strip metal, apparatus as shown in FIG. 2 has been proposed. In this apparatus, gas jet devices 3 are disposed opposite the peripheral parts of the cooling rolls 2 in contact with the strip 1. Each gas jet device 3 blows cooling gas onto the strip 1, uniformly across the width of the strip, to heat-treat it and thereby reduce the possibility of the strip being deformed out of standard.
The apparatus of FIG. 2 blows cooling gas onto the strip 1 uniformly in the widthwise direction whether or not the temperature distribution is uniform, and irrespective of the degree of non-uniformity. This renders the temperature distribution more uniform than the case where cooling gas is not blown. However, it will be appreciated that edge portions of the strip at higher temperatures are not cooled more. Hence, the temperature distribution widthwise of the strip still cannot be made sufficiently uniform. Further, the continuous and uniform blowing of cooling gas increases the electric power consumed by the apparatus. This is especially undesirable, in that the cost of production is increased and yet there is still an insufficient uniformity of the temperature distribution.